Climate change in the Amazon includes the intensification of the hydrological cycle in association with the increase of extreme events. This study aims to identify the impacts on groundwater (GW) within a pristine micro-scale catchment in the Central Amazon related to the observed alterations on hydrological cycle. Precipitation and GW levels from seven piezometers distributed along a hydrological transect covering three zones (lowland, slope and plateau) were collected for the period 2001-2021. External (evapotranspiration, GW storage and climate patterns) and computed (annual recharge) data were used to strengthen the analyses. We identified a generalized growth trend in all compartments (rainfall, evaporation and water table). There was a heterogeneity in water table fluctuations and annual recharge, depending on distance from creek channel and water depth. Surface processes influenced shallows piezometers associated with rapid overflow and low recharge. Conversely, deep piezometers involved slower processes related to water movement, and were more representative of regional GW. Those presented larger seasonal and inter-annual fluctuations, annual recharge and positive trend than shallow piezometers. Besides, all piezometers showed large inter-annual variations in recharge. The El Niño-Southern Oscillation (ENSO) influenced GW level, recharge and storage: positive phase contributed to lower all variables, with recovery in during neutral and negative phase. The larger positive trends in the deeper piezometers are a sign of the resilience of the Alter do Chão aquifer, subject to anthropogenic pressure. As water table play a key-role in shaping the structure and productivity of Amazon forests, further hydro-ecological studies should be conducted to gather information about the fate of GW-dependent Amazonian ecosystems.

Tropical rivers constitute a major portion of the global aquatic C flux entering the ocean, and the Rio Negro is one of the largest single C exporters with a particularly high export of terrestrial C. We investigated the role of whitesand ecosystems (WSEs) in blackwater formation in the Rio Negro basin to develop novel constraints for the terrestrial carbon export from land to the aquatic continuum. To this end, we used ultrahigh resolution mass spectrometry (FT-MS, Orbitrap) to identify markers in dissolved organic carbon (DOC) from ground- and surface waters of two contrasting WSEs feeding Rio Negro tributaries, and compared them with known Rio Negro marker from two openly available FT-MS datasets. Tributaries were fed by a whitesand riparian valley connected to terra firme plateau, and a typical upland whitesand Campina. WSE-DOC molecular composition differed by 80% from plateau DOC, which was characterized by reworked, highly unsaturated N- and S-containing molecules. WSE-DOC contained mainly condensed aromatics and polyphenols. WSE samples differed by 10% in molecular DOC composition and also by their isotopic content (14C, 18O, 2H). Upland WSE-DOC was exported by fresh precipitation and had maximum age of 13 years, being five years older than riparian valley WSE-DOC. Unexpectedly, only markers from the upland WSE, which cover a small proportion of the landscape, were identical to Negro markers. Markers of the riparian valley WSE, which are widespread and known for high DOC export, surprisingly showed lower coverage with Negro markers. Analysis of robust matching WSE markers between FT-MS datasets by Pubchem suggested well-known plant metabolites (chromenes and benzofurans) as promising candidates for targeted approaches and calibration. Our results suggest that terrestrial DOC from upland WSEs is a main source of specific blackwater molecules missing in the regional ecosystem C balance, whereas C export from the riparian valley and especially from terra firme plateaus represents mainly recycled and transformed carbon not directly affecting the ecosystem C balance. Our study highlights the potential of high-resolution techniques to constrain carbon balances of ecosystems and landscapes. Comparisons of FT-MS datasets and complementary isotopic information shows high potential to identify robust molecular markers that link forests, soils, aquifers and aquatic systems, and are needed for a deeper understanding of the regional C cycle in tropical blackwater catchments.